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A Houston-area storm in 2009, one of the most intense and localized in recent memory, put the 50-year-old Addicks Reservoir to the test. Is this structure and other parts of the Harris County flood control system adequate for land-use and climate changes that will occur in the next half-century?
The rain began again shortly after midnight on April 28, 2009. It was the third time that month, starting on April 18, that the western end of Harris County, Texas, had experienced excessive rainfall after a relatively dry period earlier in the year. A large thunderstorm, originating in northwest Texas, had entered the area late in the afternoon of Monday, April 27. The storm remained over the Katy, Texas, area, pounding it with a heavy rainfall of 4–5 inches, resulting in extensive street flooding and complete ground saturation before moving offshore just before midnight.
Shortly after midnight on April 28, a second wave of storms developed over the area and remained nearly stationary until it weakened and moved off over the central part of the county. Heavy rains fell between 4:00 p.m. and 10:00 p.m. on April 27 and from midnight through 8:00 a.m. on April 28, producing a total of 9.0–11.5 inches of rain across the upper portions of the Addicks Reservoir watershed. Figure 1 shows a radar estimate of rainfall amounts over the Addicks watershed on April 28.
The highest short-duration period of rainfall occurred over the Addicks Reservoir itself and parts of Buffalo Bayou. Recorded at Addicks Dam, the maximum 30-minute rainfall was 2.9 inches, which is just short of the 100-year frequency. One-hour rainfall values were measured at 4.2–4.8 inches over Addicks Reservoir as well as over upper Brays Bayou and Upper Buffalo Bayou. These rain totals are somewhat higher than the 100-year storm frequency. Six-hour and 24-hour rainfalls over Addicks were in the range of a 10-year to 50-year event.
The Addicks Reservoir watershed spans 136 square miles or 87,040 acres. If we take the average rainfall on April 27 and 28 to be 10 inches over the entire watershed, the amount of rain that fell on the watershed during that event totaled 72,500 acre-feet. This value is dependent on the estimated rainfall derived from Figure 1b and does not include any contribution from neighboring watersheds that may have overflowed.
On April 30 and May 1, immediately after the rain event, the Addicks Reservoir contained a storage content of 60,190 acre-feet of water. The storage content on April 27, before the event, was measured at 5,860 acre-feet. Thus 54,330 acre-feet of runoff was collected from the watershed during the two-day storm, which was approximately 75% of the total precipitation. It is difficult to assign an error to this number, but it is probably within ±10%. It is also a quite reasonable runoff value for such an intense storm with high precipitation rates (Smith et al. 2000). However, each storm is unique and may have a different value for the runoff fraction. In general, the runoff fraction is dependent on the degree of saturation of the soil as well as the porosity of the surface. In addition, the runoff fraction depends on the rate and duration of the precipitation. In general, the more saturated the soil and the heavier the downpour, the greater is the runoff fraction.
Even though over 2,000 homes were flooded during the April 2009 storm, the floodwater was mostly due to overflow along lower portions of creeks feeding into the Addicks Reservoir and along Buffalo Bayou and its tributaries as well as the upper Brays Bayou. The reservoir did exactly what it was designed to do.
But can the reservoir be expected to work as well in future intense rainfall events?
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